1. Field of the Invention
The present invention relates to a process for forming a reflective light valve, and, in particulars to a process for forming a reflective light valve that substantially removes wafer bowing that can degrade light valve performance.
2. Description of the Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These conventional high density projection-type color display devices typically include a light source which emits white light. Dichroic mirrors separate the white light into its corresponding red, green and blue (RGB) bands of light. Each of these colored bands of light is then directed toward a corresponding liquid crystal light valve which, depending upon the image to be projected, either permits or prevents transmission of light therethrough. Those RGB bands of light which are permitted to be transmitted through the light valves are then recombined by dichroic mirrors or a prism. A projection lens then magnifies the recombined light bands and projects the image onto a projection screen.
FIG. 1 illustrates a cross-sectional view of adjacent pixel cell structures that form a portion of a conventional light valve. Portion 100 of the conventional light valve includes a glass top plate 102 bonded to an interconnect structure 104 by a sealing member (not shown). The sealing member serves to enclose a display area and to separate glass plate 102 from interconnect 104 by a predetermined minute distance. Thus, the light valve has a cell gap 106 defined by the glass plate 102 and interconnect 104. Liquid crystal material 111, such as polymer dispersed liquid crystal (PDLC), is sealed in cell gap 106.
In a reflective mode display technology, an image is generated by creating regions within the light valve having differing contrast. This contrast is created by the state of the liquid crystal material above the reflective surface, which in turn regulates the amount of light passing from the ambient to the reflective surface.
During operation of the light valve shown in FIG. 1, selective application of voltage to pixel electrodes 112a and 112b from underlying capacitor structures 118a and 118b (through metallization 122 and 124 and via 140) switches pixel cells 110a and 110b on and off. Voltage applied to pixel electrodes 112a and 112b varies the direction of orientation of the liquid crystal material overlying the pixel electrode. A change in the direction of orientation of the liquid crystal material at the pixel electrode changes the optical characteristics of the light traveling through the liquid crystal.
If the light valve contains twisted nematic crystal, light passes through the light valve unchanged when no voltage is applied to the pixel electrode, and the light is polarized if a voltage is applied to the pixel electrode. If the light valve contains PDLC, light passes through the light valve unchanged when a voltage is applied to the pixel electrode, and light is scattered if no voltage is applied to the pixel electrode.
One key attribute of light valve performance is the amount of light reflected by the pixel cell. The degree of reflectance of the pixel cell in turn affects other system attributes such as contrast ratio, pixel coherence and brightness efficiency.
One approach to enhancing the performance of any reflective mode light valve is to increase the reflectance of the pixel electrode toward the ideal. The process for achieving maximum attainable reflectance is described in U.S. patent application Ser. No. 09/136,627, filed Aug. 19, 1998 and entitled "Silicon Interconnect Passivation and Metallization Process Optimized to Maximize Reflectance." Briefly, this process flow is optimized to eliminate and prevent roughness in the metal pixel electrode layer caused by processing that occurs after deposition of the metal pixel electrode layer.
One process that can lead to degraded reflectance in a light valve is stripping of photoresist layers that cover the pixel electrodes during fabrication of the light valve. This stripping is performed utilizing a plasma ash. The harsh conditions of the plasma ash may etch or roughen the surface of the metal pixel electrode, reducing its reflectance.
Therefore, there is a need in the art for a process flow for forming a silicon light valve that minimizes exposure of the pixel electrode to plasma ash processing steps.
A second processing step that can degrade reflectance of a pixel cell of a light valve is the deposition of polysilicon. During fabrication of the light valve, a number of polysilicon deposition steps are ordinarily performed. These poly deposition steps include formation of polysilicon elements of the capacitor structures 118a and 118b shown in FIG. 1, which control activation of the various individual pixel cells of the array.
Because polysilicon may be deposited on both sides of a wafer during deposition, one consequence of polysilicon deposition is accumulation of polysilicon on the backside of the wafer. The compressive force exerted by the backside polysilicon can cause the wafer to bow. The resulting distortion in wafer shape can degrade the reflectance of the light valve by creating a cell gap of varying width between the reflective electrode and the top glass plate. Such a nonuniform cell gap can give rise to unwanted interference fringes due to the uneven path length traversed by incident light.
Therefore, there is a need in the art for a process flow for forming a silicon light valve that minimizes the bow in the wafer bearing the silicon light valve, thereby ensuring a uniform cell gap.